A green and innovative approach to separate hexafluorophosphate from wastewater

Effluent derived from spent lithium-ion batteries (LIBs) electrolyte or discarded ionic liquids (ILs) is a potentially resource of hexafluorophosphate (PF) which can be recovered through solvent extraction. In this study, an efficient extraction approach was proposed to recover the hexafluorophosphate from solutions. A basic investigation on the interactions between extractants and hexafluorophosphate was firstly carried out using density functional theory (DFT), which demonstrated the stability of the extracted complex structure. The effects of extractant (Alamine336) and modifier, common anions and metallic ions in waste-streams, initial pH, and temperature on the extraction efficiency were explored. The results showed that 96.16 % of hexafluorophosphate could be extracted and transferred into organic phase using Alamine336/modifier as extractant under optimal conditions (0.15 mol/L of Alamine336 and 0.25 mol/L of modifier). The organic phase exhibited an excellent extraction capability of >95 % under optimal condition. Moreover, it was found that the use of sodium hydroxide as stripping reagent presented a superb stripping and cycling properties. Thermodynamic analysis and spectral data indicated that PF was extracted into the organic phase in the form of RNH⋅PF, which is a spontaneous exothermic process. The novel extraction method provides a broad application perspective for the removal and recovery of valuable hexafluorophosphate from aqueous system.